Make so EffSc can be consumed without Spongefish

[z-tech]

Summary

Splits the spongefish Cargo feature out of arkworks so downstream crates can consume effsc with arkworks fields but no Fiat-Shamir backend. Adds the missing VerifierTranscript blanket on spongefish::VerifierState (symmetric to the existing ProverTranscript blanket) so consumers no longer need a hand-written verifier adapter. Drops the now-redundant SpongefishTranscript wrapper struct.

Breaking change

effsc::transcript::SpongefishTranscript is removed — replace SpongefishTranscript::new(state) with the raw state (the blanket impl makes &mut ProverState<H, R> directly usable wherever &mut impl ProverTranscript<F> is expected).

Changes

• Cargo.toml — spongefish is now its own feature (implies arkworks); arkworks no longer pulls spongefish. Promotes rand_core from a transitive to a direct dep so sanity.rs can name RngCore without going through ark-std.
• src/transcript/mod.rs — ungates sanity; gates spongefish module on the new feature; drops the SpongefishTranscript re-export.
• src/transcript/sanity.rs — F: ark_ff::Field → F: SumcheckField, R: Rng → R: RngCore, F::rand(rng) → F::from_u64(rng.next_u64()). Now compiles under --no-default-features.
• src/transcript/spongefish.rs — deletes the wrapper struct and its inherent methods; adds the VerifierTranscript blanket on VerifierState<'a, H>.
• src/coefficient_sumcheck.rs — test passes raw &mut prover_state instead of wrapping it.

Test plan

• cargo build --no-default-features --features=arkworks succeeds; cargo tree confirms no spongefish in the dep graph
• cargo build --no-default-features succeeds
• cargo build (default features) succeeds
• cargo test --all-features — 128 tests pass
• cargo clippy --all-features --lib clean (pre-existing lints in test files unchanged)